Boomerang Seamount: the active expression of the Amsterdam–St. Paul hotspot, Southeast Indian Ridge

During a survey of the axis of the Southeast Indian Ridge (SEIR), we discovered a 1100 m tall, volcanically active submarine volcano, Boomerang Seamount, near the spreading center on the Amsterdam–St. Paul (ASP) Plateau. The summit of the volcano is 650 m below sea level and has a 200-m-deep, 2-km-wide circular caldera. Samples of very fresh volcanic glass, dated by the 210Po–210Pb technique at ∼5 months old, were collected from the floor of the caldera in March 1996. The volcano is 18 km northeast of Amsterdam Island near the intersection of a long spreading segment and the Boomerang Transform Fault. It is built on the stationary Antarctic Plate, where widely scattered volcanic activity thickens the crust, continuing to build the plateau. Water column profiles reveal a 1.7°C temperature anomaly and a 0.3 V stepped nephelometer (water column turbidity) anomaly within the caldera, nearly an order of magnitude larger than other hydrothermal plume anomalies we measured. These anomalies suggest hydrothermal activity within the caldera. Volcanic glass compositions at two sample sites on the volcano summit are similar to each other and to Amsterdam and St. Paul Island basalts, but have some important differences as well. K2O/TiO2 ratios in Boomerang Seamount glasses are similar to St. Paul Island samples, but differ significantly from Amsterdam Island samples. Rare earth element patterns in lavas from Boomerang, Amsterdam, and St. Paul are similar. Sr, Nd, and Pb isotope ratios in samples from the Boomerang Caldera floor are similar to samples from Amsterdam Island. However, another sample from Boomerang Seamount deviates from a SEIR–St. Paul–Amsterdam mixing trend and shows evidence of mixing with a Kerguelen-like source component. The geochemical complexity of these three closely spaced volcanic edifices on the ASP Plateau suggests that the Boomerang Seamount source is heterogeneous on a very small scale.